This invention concerns a continuous casting method with rollers and the relative device, used in the field of continuous casting to cast continuous steel strip by means of a pair of rollers.
To be more exact, the invention concerns a casting method with rollers wherein magnetic type means are provided with the function of laterally containing the liquid metal, the holding means cooperating with the ends of the rollers and being suitable to partly surround the curved surfaces of the rollers.
The invention is preferably applied to continuous casting equipment with rollers where the product is extracted upwards.
In the field of continuous casting of plane products such as strip or sheet, the state of the art includes the technique of casting with rollers, wherein the metal is unloaded onto a pair of counter-rotating and cooled rollers which define the gap through which the plane product transits and is extracted.
The state of the art also includes the provision of cooled and counter-rotating rollers partly immersed inside a receptacle, for example a tundish or similar, inside which the molten metal is fed; these rollers cause the product to be extracted upwards.
This casting technique with rollers has been subjected over the years to long and thorough studies, since it has a considerable potential to produce high quality strip and sheet, with high productivity, at a relatively low cost and using less manpower than traditional technologies. However, a plurality of problems connected to the technological process has not yet permitted a widespread development and diffusion of this technology on an industrial and commercial level, despite the good results obtained on the experimental level.
One of the main problems is the lateral containment of the molten metal in proximity or in correspondence with the ends of the rollers. Attempts to use holding means of a mechanical type have come up against the problem of finding materials which have at the same time low heat conductivity and the capacity to be rapidly heated to prevent the cooling and solidification of the steel on said holding means.
Moreover, the materials have to be highly resistant to wear caused by continuous contact with the rotating rollers.
Because of these difficulties, alternative holding systems of a magnetic type have been proposed. On the one hand, these have shown good results, since they allow to contain the metal efficiently without the limits of mechanical holding means; on the other hand, they have shown problems of overheating which impede a correct solidification of the strip in correspondence with the edges. These overheating problems mainly derive from using high frequency magnetic fields, which are moreover necessary since using low frequency magnetic fields would entail too high and unacceptable a turbulence in the metal during extraction.
Another technological problem concerning the continuous casting process with rollers is linked to the need to keep the free skin of the liquid metal (the meniscus) as flat and undisturbed as possible.
This is because variations in the surface of the meniscus affect the uniformity of the solidification, and therefore the thickness of the solidified skin of the strip, and can lead to breakages and cracks in the skin itself, particularly in the casting of steels with a high carbon content. This can lead to a poor quality of the steel produced.
A further problem is that it is impossible to translate the electromagnetic holding means along the axis of the rollers to vary the width of the strip produced.
In the light of all these problems, the present Applicant set himself the objective of finding an efficient solution which could be achieved industrially to obtain a device with rollers suitable to overcome the shortcomings of the state of the art. He has therefore devised, tested and embodied this invention.
The invention is set forth and characterized in the respective main claims, while the dependent claims describe other characteristics of the main embodiment.
The purpose of the invention is to obtain a continuous casting method and device with rollers, said device being provided with lateral containing means of an electromagnetic type for the molten metal, the containing means being configured in such a manner that they do not cause overheating problems to the edges of the strip produced and at the same time do not cause excessive turbulence in the bath; therefore they do not cause problems to the correct formation of the solidified skin.
Another purpose of the invention is to use electromagnetic lateral containing means which allow to create an air interspace between the liquid metal and the edge of said means, and thus prevent the formation of solidified skin on the walls of the means near the edges of the rollers.
A further purpose is to allow the lateral containing means to be moved, in order to regulate the width of the strip produced.
According to the invention, the continuous casting device comprises electromagnetic holding means, curved in shape, arranged in cooperation with each of the ends of the two counter-rotating rollers which define the extraction gap of the cast product.
The electromagnetic holding means are configured so as to surround, without contact, the surface of the rollers which are partly immersed in the molten metal, and are suitable to exert an action of lateral containment such as to prevent the spilling of the molten metal beyond the limit defined by their position.
According to one characteristic of the invention, the holding means are movable parallel to the axis of the casting rollers, so as to vary in a desired manner the width of the strip cast. According to another characteristic, the variation in the width of the cast strip can be obtained even without interrupting the casting process.
The electromagnetic holding means comprise at least a coil wound around a core, along which an alternating current of a desired intensity is made to circulate, suitable to generate a magnetic field.
The magnetic field is suitably concentrated towards the inner edges of the extraction gap defined between the casting rollers, and is suitable to induce in the molten metal the circulation of a secondary current having a direction opposite that of the primary current circulating in the coil.
The interaction between the secondary current and the components of the magnetic field induced by the primary current generates an electromagnetic force which prevents the spillage of the molten metal from the zone between the counter-rotating casting rollers.
According to one characteristic of the invention, the coil induces in the liquid metal a secondary current which has a particularly high density in the region near the coil itself. In this way, a high Joule effect is generated which prevents the formation of skin around the coil without preventing the creation of skin on the casting roller.